Systems for aligning locking screws in intramedullary nails to secure fractured bones together are known. These systems may be broadly categorized into three classes: x-ray imaging systems, mechanical systems, and electromagnetic systems. X-ray imaging systems use x-ray imaging to provide an image of the limb being treated with the inserted intramedullary nail so the surgeon may view the transverse hole located in the nail. This image facilitates the surgeon's locating the proper position on the external surface of the bone for drilling and aligning the drill bit with the transverse hole. Once the correct drill position and alignment are determined, the x-ray imaging system is removed so the surgeon may then drill a hole through the bone that passes through the hole in the nail. These x-ray imaging systems expose the patient and the surgeon to x-rays and the accumulation of x-rays, especially for the surgeon, may have long-term detrimental consequences.
The mechanical systems require reference points so the offset distance from the reference point may be externally determined and viewed by the surgeon to correlate a path through a bone to the opening of the hole in the intramedullary nail. Studies have shown, however, that an intramedullary nail may undergo some lateral and dorsal deformation as well as some rotational movement. Mechanical systems are not able to track these movements accurately and inconsistent targeting may occur as a result.
Systems that have previously used electromagnetic or magnetic components for aligning a drill for boring a hole in a bone so the drill bit passes through the transverse hole suffer from a number of limitations. Some systems of this type require that a magnet be mechanically located within the transverse hole of the nail. A pivotally mounted magnet is placed on the bone surface and moved until the magnet aligns with the dipole within the nail. This position may then be marked for drilling, but the angular orientation of the drill must be maintained by the surgeon without further reference to the external dipole that was removed for the drilling operation.
Other electromagnetic systems, such as the one disclosed in U.S. Pat. No. 5,584,838 or U.S. Pat. No. 4,621,628, use one or more electromagnetic drive coils and a plurality of electromagnetic flux sensors to guide alignment of a drill bushing with the transverse hole in an intramedullary nail. These systems measure the current or voltage induced in magnetic pick up coils associated with a drill bushing by a drive coil that is located within a medullary canal to determine the alignment of the drill bushing axis with the axis of the transverse hole. The design, development, and manufacture of these systems, however, are difficult. Additionally, some of these systems require the drive coil to be removed from its location within the transverse hole so that the drilling operation may be performed without boring through the drive coil. When the drive coil is removed from the transverse hole the coil sensors no longer generate signals that may be used to align the drill bushing. Consequently, the surgeon must maintain the proper orientation and placement of the drill without any indicia to confirm correct placement of the drill.
A system that addresses some issues arising from the use of electro-magnetic targeting devices is disclosed in published application US 2005/0075562. The system in this published application uses a permanent, cylindrical magnet that is mounted to the end of a rod so the longitudinal axis of the magnet is aligned with the rod. The magnet is designed to have a magnetic field that is axisymmetric. Such a magnet is made by polarizing a cylindrical magnet through its diameter rather than along its longitudinal axis. The magnet is placed within an intramedullary nail at a position just short of a transverse hole in the nail. An elliptical array of magneto-resistive (MR) elements is mounted in fixed relation to one or more drilling sleeves. The MR elements are composed of material that changes its electrical resistance in response to magnetic flux passing through a sensor element. The MR elements are coupled together in a Wheatstone bridge arrangement so that the voltage output of the bridge is zero when the array is in a position where the plane of the sensor array is parallel to the plane through the longitudinal center plane of the magnet and the center of the sensor array is over the center of the magnet.
The system in the published application suffers from the use of the MR elements. While these elements are sensitive to changes in magnetic flux, they are not always consistent in their responses. Specifically, these elements have a tendency to experience hysteresis. Consequently, an MR element may produce one resistance at a given magnetic field strength and then produce a different resistance at the same magnetic field strength as the magnetic field strength is varied between measurements. Variations in magnetic field strength are common as a surgeon moves and rotates the sensor array to locate the zero point. In order to compensate for any hysteresis experienced by the magnetic sensor elements, the sensor array is frequently reset.
Frequent resetting also confirms that the reading generated by the sensor array arises from the magnetic field generated by the magnet within the intramedullary nail. Environmental magnetic fields may affect the reading generated by the sensor array because MR elements are sensitive to very small changes in magnetic field strength. Resetting the array is thought to be beneficial because as the sensor array is brought closer to the bone, the magnetic field generated by the magnet in the bone dominates. The earth's magnetic field, however, does vary as a function of time and spatial orientation in the vicinity of the patient's bone. Consequently, it may affect the reading generated by the sensor array even when the array is reset.